The present disclosure relates generally to nuclear well logging and, more particularly, to techniques for identifying gas in certain formations, such as shaly sands.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Nuclear downhole tools are frequently used in the oilfield to determine the properties of a subterranean formation. The valuable information gathered by nuclear downhole tools may indicate, for example, the location and concentration of hydrocarbons such as oil and gas, as well as other properties such as the density or porosity of the subterranean formation. In general, nuclear downhole tools operate by emitting some form of nuclear radiation (e.g., neutrons or gamma rays) into the formation surrounding a borehole. The emitted nuclear radiation interacts with the elements of the formation, the results of which can be detected by nuclear radiation detectors (e.g., neutron detectors or gamma ray detectors) in the downhole tool. Properties of the subterranean formation can then be determined based on the amount and type of radiation detected by the nuclear downhole tool.
Nuclear downhole tools are generally classified as wireline tools or logging-while-drilling (LWD) tools. Wireline tools may be lowered into a borehole to obtain measurements after the borehole has been drilled and/or cased with a casing. Thus, at the time of measurement, materials other than the formation itself may obscure the measurements of the downhole tool. For example, by the time a wireline tool obtains measurements of a subterranean formation, the borehole and surrounding formation may have become invaded by drilling fluid or by hydrocarbons. On the other hand, LWD tools may obtain measurements of the subterranean formation in an openhole reading at the time the borehole is initially being drilled. Since LWD tools take measurements of the formation at the time the borehole is being drilled, fewer materials other than the subterranean formation affect the measurement.
Both wireline and LWD nuclear downhole tools that perform pulse neutron capture (PNC) measurements have been developed. In general, PNC measurements involve emitting pulses of neutrons into the surrounding formation to be “captured” by the nuclei of elements of the formation. When the nuclei capture the neutrons, they emit gamma rays as a result. By measuring the extent to which these capture gamma rays are detected by radiation detectors in the downhole tool, a “capture cross-section” of the formation can be obtained. The capture cross-section of the formation is also referred to as the sigma measurement, and is used to discriminate between hydrocarbons and saline water in the subterranean formation, since chlorine in the salt water has a very large capture cross-section compared to hydrocarbons and reservoir rocks. The greater the total salt count (NaCl per 1,000 ppm) in the water contained by the subterranean formation, the better a PNC tool may quantitatively describe the water saturation.
In certain formations such as shale, sandstone, dolomite, and/or carbonate, however, the sigma measurement may not always accurately indicate certain formation properties. In fact, many large reserves of hydrocarbons in the Gulf of Mexico and elsewhere may have many zones of with significant amounts of shale and other similar rocks. It is believed that some prospects in these reserves apparently looked qualitatively marginal, or even bad, due to the effects of excess shale on PNC measurements. Many of these zones therefore may have been passed up indefinitely or, worse yet, condemned as non-productive.